Atomic resonance device using composite vacuum envelope and pump apparatus



Oct. 11, 1966 H. E. PETERS 3,278,857

ATOMIC RESONANCE DEVICE USING COMPOSITE VACUUM ENVELOPE AND PUMP APPARATUS Filed April 6, 1964 FEGE I NVENTOR. HARRY E. PETERS ATTORNEY United States Patent 3,278,857 Patented Oct. 11, 1966 ice 3,278,857 ATOMIC RESONANCE DEVICE USING COMPOSITE VACUUM ENVELOPE AND PUMP APPARATUS Harry E. Peters, Beveriy, Mass, assignor to Varian Associates, Palo Alto, Calif, a corporation of California Filed Apr. 6, 1964, Ser. No. 357,722 5 Claims. (Cl. 331-94) The present invention relates in general to atomic resonance devices and more particularly to improved resonance beam tubes having a composite vacuum pump and envelope portion. Such atomic beam tubes are especially useful as frequency standards, frequency stabilizers and as sensitive magnetometers.

Heretofore hydrogen maser beam tubes have been built wherein a beam of atomic hydrogen in an upper hyper fine energy state was projected over an elongated beam path into a resonant cavity. The resonant cavity contained a bounce box for storage of hydrogen atoms in an upper hyperfine energy state for a time on the order of 1 second such that the stored hydrogen particles could undergo a hyperfine transition giving off stimulated emission of radiation to produce maser oscillation. The maser oscillation is coherent and has extremely high spectral purity. The maser oscillating signals are withdrawn from the cavity and may be utilized for a frequency standard, frequency control or the like. Such a hydrogen maser is described in US. patent application Ser. No. 142,356 titled, Atomic Hydrogen Maser, inventor, Norman P. Ramsey et al., filed October 2, 1961, and assigned to the same assignee as the present invention.

Such prior art hydrogen maser tube structure have generally included an elongated vacuum envelope partitioned into two sections, an upstream section and a downstream section. The downstream section of the envelope included the storage bulb and maser cavity. This section of the envelope was operated at a pressure on the order of 10* millimeters mercury to prevent pertubation of the hyperfine atoms by collision with other gaseous atoms or molecules which collisions would adversely affect the resonant frequency of the maser oscillation. The second or upstream portion of the vacuum envelope includes a state selecting magnet and collimator wherein 99% of the initial beam particles are discarded and is generally operated at a pressure of on the order of two magnitudes higher than the pressure within the downstream portion of the envelope. Two separate vacuum pumps were provided in the prior maser, one pump being connected via a suitable T-shaped tubulation or manifold into each of the separate sections of the elongated vacuum envelope portion.

One of the problems with this prior art maser which utilized two separate vacuum pumps was that the vacuum pumps and their connecting manifolds were lead off at right angles to the elongated vacuum envelope of the maser such that the pumps which were relatively heavy could hang down and be supported from the exhaust manifold structures or be supported by separate support structures. Such manifold and support structures were relatively bulky and costly.

In the present invention a vacuum pump structure is employed which includes a hollow chamber with plurality of pumping elements radially projecting out from the chamber portion. The pump chamber forms also a portion of the maser vacuum envelope such that separate vacuum envelopes are not required for the maser and for the vacuum pump. In addition, separate support structures for the vacuum pumps are eliminated by use of the composite vacuum envelope and pump structure.

The principal object of the present invention is the provision of an improved atomic resonance beam tube having a composite vacuum envelope and pump structure whereby the size, weight, complexity and cost of the tube are reduced.

One feature of the present invention is the provision of an atomic beam tube structure including a composite vacuum envelope portion and pump structure wherein the pump includes a hollow chamber axially coextensive with and surrounding the beam path and forming a portion of the tube envelope with a plurality of pumping elements projecting outwardly from the chamber in gas communication therewith whereby the tube structure is greatly simplified.

Another feature of the present invention is the same as the preceding feature including the provision of a septum subdividing the pump chamber into an upstream and a downstream portion with separate pumping elements directly communicating with each of the separate portions of the subdivided chamber, whereby the separate chamber portions of the pump can be operated at substantially different pressures in use.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a longitudinal sectional view of a hydrogen maser atomic beam tube apparatus employing features of the present invention, and

FIG. 2 is a transverse sectional view of the structure of FIG. 1 taken along lines 22 in the direction of the arrows.

Referring now to FIG. 1, there is shown in partial crosssectional view the hydrogen maser apparatus of the present invention. The apparatus includes a beam generating device 1 for forming and projecting a 'beam of atomic hydrogen generally longitudinally of the tube apparatus. A cavity resonator 2 is disposed at the terminal end of the beam path. The cavity resonator 2 contains therewithin a storage bulb 3 or bounce box of Pyrex or glass with a non-relaxing wall coating as of Teflon for storing the atomic hydrogen particles for on the order of one second duration before they exit from the bulb 3 through the entrance port. Within the bulb 3 the stored particles undergo a hyperfine transition at about 1420 megacycles giving off coherent spontaneous emission of radiation which is extremely stable in frequency.

A state selecting hexapole magnet 4 is disposed at the up stream end of the atomic beam path for focusing out of the beam the hydrogen atoms that are not in the desired upper hyperfine energy state. An elongated tube vacuum envelope structure 5 surrounds the cavity and the beam path inbetween the source 1 and the cavity 2. A vacuum pump 6 surrounds the initial portion of the beam path and forms a combined envelop portion and vacuum pump 6.

The pump 6 is of the getter ion type described in US. Patent 2,983,433 issued May 9, 1961. The pump 6 contains a hollow rectangular central chamber 7 defined by the vacuum envelope of the pump. A plurality of smaller outwardly extending longitudinally directed pumping chambers 7' axially extend the length of the chamber 7 and communicate with the central chamber 7 and are again defined by outward projections of the vacuum envelope of the pump 6.

Pumping elements are disposed within each of the longitudinally directed smaller pumping chambers 7'. The pumping elements include a pair of spaced apart cathode plates 8 disposed on opposite sides of a cellular anode structure 9. Permanent magnets 11 are preferably disposed externally of the pump envelope and develop gaps of magnetic field. The magnetic field lines within the gaps thread through the cathode plates 8 and cellular anode 9 with the magnetic field lines being coaxially disposed of the anode cells. The field lines pass through the 3 cathode plates substantially perpendicularly thereto for enhancing the intensity of the glow discharge of multiple glow discharge columns passing through the cellular anode 9 between the cathode plates 8.

A semi-frusto-conical septum 12 is disposed diagonally across the chamber 7 dividing the pump into an upstream portion 10' containing 3 of the lesser chambers 7' and a downstream portion 10" containing one of the lesser chambers 7. The septum 12 is centrally apertured and carries therefrom an elongated small diameter bore tube 13 as of A1" diameter with its bore in axial alignment with the beam path. The tube 13 is sealed over an aperture 14 in the septum 12 via the intermediary of a support tube 15 of slightly larger diameter than the narrow bore tube 13. The small diameter bore tube 13 is for the purpose of allowing the beam to pass through the septum 12 while permitting the pressure in the downstream portion 10" of the chamber 7 to be operated at a pressure substantially below the pressure of the upstream portion 10' of the envelope since the small bore ofiers every high impedance to gas flow therethrough. In addition, the small bore tube 13 serves as an additional beam collimator for eliminating certain beam particles emerging from the state selecting magnet 4. The particles eliminated are not in the proper state and have been slightly deflected out of the desired beam path. A stopping bead 20 is carried in axial alignment with the center of the beam slightly downstream of the hexapole state selecting magnet 4 for stopping all beam particles on the center line of the beam which therefore had not seen the strong focusing magnetic fields of the hexapole magnet 4. In a typical example about 1% of the total atomic particle beam flux emerging from the source 1 passes through collimator tube 13. Therefore, 99% of the initial beam flux is pumped from the upstream chamber of the pump 6.

High voltage feed through insulator assemblies 16, disposed at opposite ends of the pump 6, supply operating potentials as of +2000 v. to the anodes 9 of the pumping elements with respect to the grounded pump envelope and cathode plates 8. A pair of separate conventional pump power supplies, not shown, are connected to the separate feed through insulator assemblies 16 at opposite ends of the pump such that the pump 6 is effectively divided into two pumps to operate the upstream portion 10 of the chamber at a substantially different pressure than the downstream portion 10" of the chamber 7.

The pump is provided with a pair of axially aligned cylindrical tubes 17 communicating with the central chamber 7 at opposite ends thereof and form extensions of the vacuum wall of the pump. The tubes 17 are provided with outwardly directed vacuum-tight flanges 18 for coupling to similar mating flanges. One of the punrp flanges 18 is sealed to a similar mating flange 19 carried on one end of the reduced neck portion of the central portion of the tube envelope. The other pump flange is sealed to a similar flanged subassembly containing the source 1. The flanged subassembly comprises a re-entrant tubular support member 21 coaxially disposed of the tubular portion of the pump envelope. The support member 21 carries the hexapole magnet 4 from the inner end thereof with the hexapole magnet 4 axially coextensive with a lower portion of the pump chamber 7.

A pair of magnetic shields 22 as of sheet soft iron and A sheet Mu metal, respectively, surround the vacuum pump 6 for shielding the cavity 2 from stray magnetic fields produced by the permanent magnets of the pump 6.

The storage bulb 3 is supported within the cavity resonator 2 via the intermediary of a dielectric tube 23 with its axial bore in coaxial alignment with the hydrogen beam path for passage of the beam therethrough. The bulb 3 is provided with an aperture 24 in axial alignment with the tube 23. Multiple hole collimator 25 is disposed in a narrow neck portion of tube 23. Atomic hydrogen beam particles pass through the tube 23, col limator 25, aperture 24 and into the bulb 3.

Within the bulb the beam particles undergo many successive wall collisions as of more than 10,000 before they finally pass out of the bulb via aperture 24, collimator 25 and tube 23. The collimator 25 comprises a bundle of 25 axially aligned glass tubes 2" long and inside diameter, the bundle of tubes being approximately /2" in diameter. A dielectric support rod 26 is fixedly secured to the bulb 3 and extends away from the bulb 3 in a way that is diametrically opposed to the support tube 23. The dielectric rod 26 is supported in a spring loaded pocket 27 and together with the tube 23 supports the dielectric storage bulb 3 within the cavity 2. A coupling loop 29 is formed in the end of a coaxial line 31 and passes into the cavity 2, is coupled to the field thereof, and serves to extract the output signal of the maser and passes same to a suitable utilization device, not shown.

A hollow cylindrical oven 32 surrounds the maser cavity 2 and retains same at a precisely controlled temperature as of, for example, 40 C. This prevents uncontrolled shifts in the frequency of the maser cavity due to temperature variations of the ambient environment. A pair of spaced hollow cylindrical magnetic shields 33 which are closed at their ends coaxially surround the cavity 2 to prevent stray magnetic fields from passing through the storage bulb 3. If these stray fields were not shielded they could stimulate undesired transitions and would have a second order effect upon the frequency of the maser oscillations.

A suitable cabinet 34 surrounds the lower portion of the tube and supports the tube via the intermediary of a heavy flange 35 as of aluminum fixedly secured to a reduced neck portion of the vacuum envelope 5.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An atomic resonance beam tube apparatus includ ing means forming an elongated tubular vacuum envelope structure for the atomic beam tube apparatus, means disposed at one end of said elongated envelope for projecting a beam of atomic particles longitudinally of the tube envelope and generally coaxially thereof, means disposed along said beam :path for producing resonance of the atomic beam particles and for generating an atomic resonance signal, a vacuum pump structure disposed inbetween said atomic beam source and said atomic resonance producing means and surrounding said beam path, said vacuum pump structure being in gas communication with the interior of said elongated vacuum envelope for exhausting said envelope to a substantially reduced pressure for proper operation of the atomic beam tube, said vacuum pump having a hollow chamber axially coextensive with a portion of the beam path and including [a plurality of pumping elements projecting outwardly from said hollow chamber portion, and a substantial portion of said chamber of said vacuum pump also forming a substantial portion of the elongated tubular vacuum. envelope of the atomic beam tube whereby a composite tube vacuum envelope and pump envelope structure is obtained thereby simplifying the structure of the atomic beam tube apparatus.

2. The apparatus according to claim 1 wherein said hollow chamber of said vacuum pump structure includes a partition diagonally extending across said chamber portion of said vacuum pump for dividing said pump structure into an upstream chamber and a downstream chamber, said chambers operating in use at substantially reduced pressures, and said partition including an aperture in substantial alignment with the beam path for passing the beam therethrough.

3. The apparatus according to claim 2 wherein said pump includes a flange assembly at the upstream end thereof, and wherein said beam projecting means is mounted from said flange closing ofl the upstream end of said vacuum pump structure.

4. The apparatus according to claim 3 wherein said beam projecting means is coaxially disposed of and axially coextensive with a portion of said upstream. chamber portion of the vacuum envelope structure of said pump, and wherein a magnetic state selecting magnet assembly is disposed downstream of said source in axially coextentube structure includes a similar flange member mating with said downstream flange portion of said vacuum pump.

No references cited.

ROY LAKE, Primary Examiner.

S. H. GRIMM, Assistant Examiner. 

1. AN ATOMIC RESONANCE BEAM TUBE APPARATUS INCLUDING MEANS FORMING AN ELONGATED TUBULAR VACUUM ENVELOPE STRUCTURE FOR THE ATOMIC BEAM TUBE APPARATUS, MEANS DISPOSED AT ONE END OF SAID ELONGATED ENVELOPE FOR PROJECTING A BEAM OF ATOMIC PARTICLES LONGITUDINALLY OF THE TUBE ENVELOPE AND GENERALLY COAXIALLY THEREOF, MEANS DISPOSED ALONG SAID BEAM PATH FOR PRODUCING RESONANCE OF THE ATOMIC BEAM PARTICLES AND FOR GENERATING AN ATOMIC RESONANCE SIGNAL, A VACUUM PUMP STRUCTURE DISPOSED INBETWEEN SAID ATOMIC BEAM SOURCE AND SAID ATOMIC RESONANCE PRODUCING MEANS AND SURROUNDING SAID BEAM PATH, SAID VACUUM PUMP STRUCTURE BEING IN GAS COMMUNICATION WITH THE INTERIOR OF SAID ELONGATED VACUUM ENVELOPE FOR EXHAUSTING SAID ENVELOPE TO A SUBSTANTIALLY REDUCED PRESSURE FOR PROPER OPERATION OF THE ATOMIC BEAM TUBE, SAID VACUUM PUMP HAVING A HOLLOW CHAMBER AXIALLY COEXTENSIVE WITH A PORTION OF THE BEAM PATH AND INCLUDING A PLURALITY OF PUMPING ELEMENTS PROJECTING OUTWARDLY FROM SAID HOLLOW CHAMBER PORTION, AND A SUBSTANTIAL PORTION OF SAID CHAMBER OF SAID VACUUM PUMP ALSO FORMING A SUBSTANTIAL PORTION OF THE ELONGATED TUBULAR VACUUM ENVELOPE OF THE ATOMIC BEAM TUBE WHEREBY A COMPOSITE TUBE VACCUUM ENVELOPE AND PUMP ENVELOPE STRUCTURE IS OBTAINED THEREBY SIMPLIFYING THE STRUCTURE OF THE ATOMIC BEAM TUBE APPARATUS. 